The environmental aspects of pulp and paper manufacture has been in focus for the industry during the past 15 years. Starting with efforts to reduce colour, COD and BOD in bleach effluents and SO.sub.2 in flue gases the emphasis has very much been on reducing chlorinated organics resulting from chlorine based bleaching. Chlorine dioxide bleaching has shown to be an environmentally friendly process. Also totally chlorine chemical free (TCF) bleaching has been developed. In order to reduce all environmental impact of pulp manufacture the industry is now seeking ways to "close" the mill, or in other words eliminate effluents and instead remove a small amount of waste in solid form under controlled conditions. In the totally closed pulp mill this waste will contain only the trace elements once taken up from the ground by the trees and it is the ultimate goal that these can be fed back to the forestal areas.
In Tappi J. Febr. (1989) p. 167-170 it is disclosed that lignin can be separated from bleach effluents. Acid and alkaline effluents are mixed with fibres and a cationic polymer. This gave a reduction of 41 to 67% of the content of chemical oxygen demanding constituents, measured as COD.sub.Cr.
Other ways to treat bleach plant effluents have also been described, see APPITA '91/187-191. A process is described where the effluent volume is reduced by evaporation in a low temperature evaporator and the produced condensate is recycled back to the pulping process. The remaining concentrate can be further evaporated and burnt to obtain an inorganic salt for landfilling. The evaporation of solutions with high salt concentrations is known to be difficult due to problems with incrustation and increasing boiling temperature. To obtain a total incineration of the bleach plant effluent, it is therefore necessary to combine the first evaporator with expensive and energy consuming high effect evaporators. The process need a separate furnace that can manage combustion of organics in presence of large amounts of chloride.
U.S. Pat. No. 3,986,951, assigned to Champion Int Corp., discloses an effluent treatment process for a chlorine based bleachery including organics and solids removal before electrodialysis and brine electrolysis. The effluent from the chlorine stage is treated by adding Al.sup.3+ to precipitate organics and suspended solids, filtered and then electrodialyzed to form a diluate stream containing 200 to 600 ppm NaCl which is returned to the bleachery and a concentrated stream containing 5-10 wt % NaCl. The latter stream is concentrated further by evaporation to 25 wt % and electrolyzed to form Cl.sub.2, NaOH and NaOCl. The process is said to permit reusage of water, recovery of chemicals and reduction of sludge if integrated in a pulp mill. This process treats an effluent with low concentration of organics. Thus, it is difficult to obtain an efficient precipitation of organics which increases the risk of negative effects of scaling in the electrodialysis cell. The use of aluminum makes it impossilble to recycle the obtained precipitate to the existning furnaces. At the same time the concentration of inorganic salts is very low. The electrodialysis stack must therefore be operated at a low current density and with large effluent volumes which gives large process equipment and high investment cost.